This technology is applicable to the production of crystalline silicon for photovoltaic and semiconductor applications. Semiconductor silicon has become the most important and characteristic material of the technological age. Today there are no electronic devices that do not use crystalline silicon material. Silicon, being an elemental semiconductor, does not have any interdiffusion or stoichiometry related issues as in the case of expensive III-V or II-VI compound semiconductor materials. Currently the silicon single crystals for electronic device applications are produced mainly by the Czochralski (CZ) technique and the Float Zone (FZ) technique. Because of the cost of the single crystalline silicon substrate and low production rates, industries have shifted their direction towards multicrystalline silicon even though single crystalline silicon produced by the CZ and FZ techniques yield higher solar cell efficiencies. Multicrystalline silicon is mainly grown by directional solidification, the heat exchanger method, the Bridgman method, electromagnetic casting, and conventional casting. Even though the cell efficiency of the multicrystalline material is lower when compared to its single crystalline counterpart, the shift towards multicrystalline silicon is because of the ease of production and higher throughputs.
Both in the CZ technique and the directional solidification techniques, transparent or opaque quartz crucibles for one time use are widely used in silicon crystallization processes for the semiconductor and photovoltaic industries. However, they experience a variety of material interaction problems which affect the quality of the crystallized silicon and cause failures in crystal growth process runs.
To keep the cost of crystalline silicon devices, including photovoltaic and electronic devices low, the cost of the silicon substrate must be kept low. Hence it is necessary to find an inexpensive way for growing crystalline silicon crystal.
The present invention is a novel crystalline silicon manufacturing concept, using a reusable crucible, and is expected to change the conventional manufacturing process of silicon. The crystalline silicon material grown from a reused crucible has less oxygen and carbon, which dramatically improves the device performance.
The search for a cost effective and usable crucible material for the manufacturing of crystalline silicon is an active area of research. For the growth of crystalline silicon for photovoltaic (PV) applications, slip cast silica crucibles are widely used. This type of crucible collapses after use, requiring a new crucible for each run. For example, to grow a 240 kilogram multicrystalline silicon ingot, a 69×69 cm crucible is required at a present cost of about $900 (US). The cost of the crucible is a significant contributor to the total cost of the ingot production process.
In spite of the unique beneficial properties of quartz materials, there are a number of difficulties experienced in employing quartz crucibles when the temperature of the crucible is close to or exceeds the melting point of silicon. Some of them are:                1. Silicon, when it is molten or near-molten (i.e. solidifying from the melt), is extremely reactive to the materials used to contain it. At temperatures greater than about 1300° C., silicon begins to attack and corrode such materials, deriving impurities from the crucible.        2. The mechanical and electrical properties of silicon crystals are influenced by metallic and non-metallic impurities in the silicon. Oxygen atoms incorporated in the silicon during the crystal growth process is a significant factor. The majority of oxygen in a grown silicon crystal is atomically dissolved, and occupies the interstitial sites. Oxygen-related defects in silicon cause stacking faults, thermal donor generation and oxide precipitation. In typical crystal growth systems where the molten silicon is held in contact with fused quartz crucible, the latter is the main source for oxygen, and results in oxygen concentration in the order of 5×1017 to 1×1018 atoms/cm3 in silicon. Interface reactions between molten silicon and the quartz container are important in controlling the oxygen incorporation, and thus the properties of the single crystal silicon.        3. Quartz crucibles tend to deform at high temperature because of the softness of vitreous silica at temperatures exceeding the melting point of silicon. For this reason, secondary containers such as graphite susceptors are usually used to support the fused quartz crucibles.        4. The quartz (SiO2) crucible undergoes several pertinent reactions in the system (with molten silicon; with secondary graphite containment, etc.): Such reactions and Gibbs free energies computed from JANAF (Joint Army, Navy, Air Force) thermo chemical data (NIST (National Institute of Standards and Technology) Standard Reference Data Program) are:        
                                                                                                              SiO                    2                                    ⁡                                      (                    s                    )                                                  +                                  Si                  ⁡                                      (                                          s                      ,                      l                                        )                                                              =                              2                ⁢                                  SiO                  ⁡                                      (                    g                    )                                                                                                                                                                                                    Δ                ⁢                                                                  ⁢                                  G                  T                  0                                            ,                                                cal                  /                  reaction                                =                                                      164                    ,                    340                                    -                                      (                                          79.5                      *                      T                                        )                                                                                                          (                              1000                -                                  1685                  ⁢                  K                                            )                                                                                          a                .                            =                                                148                  ,                  500                                -                                  (                                      70.0                    *                    T                                    )                                                                                        (                              1685                -                                  2000                  ⁢                  K                                            )                                                          [        1        ]                                                                                                                          SiO                    2                                    ⁡                                      (                    s                    )                                                  +                                  C                  ⁡                                      (                    s                    )                                                              =                                                SiO                  ⁡                                      (                    g                    )                                                  +                                  CO                  ⁡                                      (                    g                    )                                                                                                                                                                                                    Δ                ⁢                                                                  ⁢                                  G                  T                  0                                            ,                                                cal                  /                  reaction                                =                                                      162                    ,                    250                                    -                                      (                                          80.6                      *                      T                                        )                                                                                                          (                              1000                -                                  2000                  ⁢                  K                                            )                                                          [        2        ]                                                                                                                          SiO                    2                                    ⁡                                      (                    s                    )                                                  +                                  3                  ⁢                                      C                    ⁡                                          (                      s                      )                                                                                  =                                                SiC                  ⁡                                      (                                          s                      ,                      α                      ,                      β                                        )                                                  +                                  2                  ⁢                                      CO                    ⁡                                          (                      g                      )                                                                                                                                                                                                                                                    ⁢                                                Δ                  ⁢                                                                          ⁢                                      G                    T                    0                                                  ,                                                      cal                    /                    reaction                                    =                                                            143                      ,                      830                                        -                                          (                                              80.4                        *                        T                                            )                                                                                                                              (                              1000                -                                  2000                  ⁢                  K                                            )                                                          [        3        ]            
The CO gas generated reacts with molten Si by the reaction represented by the following formula, leaving carbon in the molten Si, which gets segregated in the solid Si ingot.CO+Si(Molten)=SiO+C  [4]
Carbon (C), typically in the order of 4 parts per million (ppm), is incorporated into the silicon ingot which is produced since the molten state of silicon is required to be maintained for a long period of time.
Silicon carbide may be formed in the silicon ingot, which deteriorates the wafer quality apart from making inclusions in the ingot posing problems during cutting processes.
At high temperatures, molten silicon reacts with quartz and during solidification the silicon and quartz adhere to each other. Consequently, due to the difference in the coefficient of thermal expansion, both the crucible and ingot crack when they cool down. Also, the difference in thermal expansion between the solidifying silicon ingot and the crucible induces stress into the portions of the ingot that are in contact with the crucible, thus creating dislocations and non-usable regions.
Solutions to the above adhesion problem encountered in directional solidification of polycrystalline silicon can be overcome by applying a protective coating layer on the inner walls of the quartz crucible. Various coatings, including oxides, nitrides, carbides of silicon and combinations thereof, have been reported. This thin layer of coating essentially acts as a release agent. Among the several materials used as the coating layer, silicon nitride is the most widely used.
Due to the non-wetting characteristics of the silicon melt combined with the above mentioned coatings, the silicon ingot could be grown free of cracks. The use of silicon nitride as a coating material is well reported in the literature. Saito et al. [Conf. Rec. of 15th PV Spec. Conf.] reported the successful growth of polycrystalline silicon ingot by employing such a coating on the inner surface of a crucible. Several different processes have been proposed for the application of the silicon nitride layer.
A wet spray method is disclosed in the prior art in which a water-based suspension of silicon nitride with binder and defoamer is spray-painted on the inner surface of the quartz crucible. The wet release coating is heated in a kiln to remove the binder. The finished coating possesses sufficient strength to maintain coating integrity during loading of the polysilicon and manipulation of the crucible into the growth furnace.
Other prior art describes the usage of silicon nitride on a silica crucible. There is also prior art that describes a silicon nitride coating process on a silica crucible. Another piece of prior art discloses a CVD coated silicon carbide for growing silicon crystals by a pulling process. Yet other prior art demonstrates the usage of hard coating of zirconates for silicon crystallization.
The use of silicon nitride coating alone has deleterious effects since the layer itself will decompose at higher temperatures, thus introducing nitrogen into the silicon melt. Secondly, since the coating is so porous it will allow the silicon melt to come in contact with the crucible walls, which are made out of silica, thereby drawing impurities from the crucible wall. In using a silica crucible, oxygen is introduced into the silicon melt by the reaction of silicon with the silica surface. Too much oxygen is not encouraged for the production of solar cells, while oxygen is needed for the fabrication of integrated devices.
Rudiger et al. (J. Electrochem. Soc. Vol. 142, 1995) have reported on the reaction of molten silicon with silicon nitride and other refractory materials. The studies clearly show that when silicon is melted in silicon nitride-coated crucibles, the silicon melt does not wet the silicon nitride for the first 20 minutes. At longer reaction times, the melt creeps through the silicon nitride coating.
Though silicon nitride and silicon oxynitride are used as coatings in large scale as crystal growth processes, as claimed by Prakash et al. (J. Cryst. Growth 144 (1994) 41), these coatings alone are not effective to achieve chemical purities for device application. The search for new coating technologies continues to receive significant attention. In order to prevent the silicon melt from coming in direct contact with the silicon nitride, researchers have also reported the use of molten salts with non-wetting characteristics.
The use of graphite as an alternative to quartz was widely attempted. Ciszek et al. in their article in IBM J. Res. Dev. have illustrated a process of growing solar grade silicon by directional solidification in carbon crucibles. Here, the graphite crucible is a sacrificial crucible, i.e. one crucible yields one run, because of the adhesion of the silicon to the crucible walls. A Ukrainian research group has also demonstrated a carbon-carbon crucible for silicon solidification.
Saito et al. (Solar Energy Materials, Vol. 9, 1983) developed a SiC coated carbon or sintered silicon nitride reusable mold with a coating of silicon nitride as the mold release agent. A CVD coated silicon carbide on a graphite mold in combination with silicon nitride coating as mold release for growing silicon crystals is also described in the prior art.
Though the above processes are suited to produce crystalline silicon, the crucible cost is more expensive than the quartz crucible.
To get rid of the impurity incorporation from the quartz crucible and to save cost on the crucible material the present invention is directed towards a process of growing crystalline silicon from non-quartz and reusable crucibles.